1. Field of the Invention
This invention relates to a non-linear optical crystal element. More particularly, it relates to a non-linear optical crystal element having an optical thin film exhibiting low reflectance with respect to ultraviolet light.
2. Background of the Invention
In the field of optoelectronics performing information processing or measurement control using laser light, a demand is raised for a shorter wavelength and a higher output of the laser light radiated from a laser light source.
In an ultraviolet wavelength range of 400 nm or less, a semiconductor laser cannot be excited into oscillations without difficulties. Thus a laser light source has been conceived which exploits the phenomenon of second harmonic generation (SHG) of the non-linear optical crystal element. This laser light source converts the wavelength of the incident light by a wavelength converter employing a non-linear optical crystal element and radiates an ultraviolet (UV) laser light a beam, that is a laser light beam in the wavelength range of ultraviolet rays. The laser light source has a non-linear optical crystal element in a ring-shaped resonator and generates the UV laser light on radiating a laser light beam as a basic wave to the non-linear optical crystal element.
On the surface of the non-linear optical crystal element is formed an optical film for reflecting the UV rays comprising a non-reflective coating film preferably formed by vacuum deposition.
For the non-linear optical crystal element, a single crystal of potassium dihydrogenphosphate (KDP) or barium borate (BBO) or the like is employed. Since the non-linear optical crystal element, such as KDP or BBO, is a deliquescent single crystal, it needs to be protected against crystal destruction otherwise caused by deliquescence. However, the thin film formed by heating of a single crystal substrate of BBO or KDP followed by vacuum deposition is generally of a cylindrical structure and water-permeable and hence cannot play the role of a protective film. That is, the optical thin film formed on the single crystal by vacuum deposition is low in atomic or molecular density and hence permits the moisture in air to be intruded into interstices between atoms deposited by vacuum deposition. The UV laser light radiated from the non-linear optical crystal element is then subjected to wavelength shift because of the difference in the refractive index between the moisture and the single crystal of KDP or BBO.